The integrins are a class of membrane-associated molecules which actively participate in cellular adhesion. Integrins are transmembrane heterodimers comprising an .alpha. subunit in noncovalent association with a .beta. subunit. To date, at least fourteen .alpha. subunits and eight .beta. subunits have been identified [reviewed in Springer, Nature 346:425-434 (1990)]. The .beta. subunits are generally capable of association with more than one .alpha. subunit and the heterodimers sharing a common .beta. subunit have been classified as subfamilies within the integrin population.
One class of human integrins, restricted to expression in white blood cells, is characterized by a common .beta..sub.2 subunit. As a result of this cell-specific expression, these integrins are commonly referred to as the leukocyte integrins, Leu-CAMs or leukointegrins. Because of the common .beta..sub.2 subunit, an alternative designation of this class is the .beta..sub.2 integrins. The .beta..sub.2 subunit (CD18) has previously been isolated in association with one of three distinct .alpha. subunits, CD11a, CD11b or CD11c. The isolation of a cDNA encoding human CD18 is described in Kishimoto, et al., Cell 48:681-690 (1987). In official WHO nomenclature, the heterodimeric proteins are referred to as CD11a/CD18, CD11b/CD18, and CD11c/CD18; in common nomenclature they are referred to as LFA-1, Mac-1 or Mol and p150,95 or LeuM5, respectively [Cobbold, et al., in Leukocyte Typing III, McMichael (ed), Oxford Press, p.788 (1987)]. The human .beta..sub.2 integrin .alpha. subunits CD11a, CD11b and CD11c have been demonstrated to migrate under reducing condition in electrophoresis with apparent molecular weights of approximately 180 kD, 155 kD and 150 kD, respectively, and DNAs encoding these subunits have been cloned [CD11a, Larson, et al., J. Cell Biol. 108:703-712 (1989); CD11b, Corbi, et al., J. Biol. Chem. 263:12403-12411 (1988) and CD11c, Corbi, et al. EMBO J. 6:4023-4028 (1987)]. Putative homologs of the human .beta..sub.2 integrin .alpha. and .beta. chains, defined by approximate similarity in molecular weight, have been variously identified in other species including monkeys and other primates [Letvin, et al., Blood 61:408-410 (1983)], mice [Sanchez-Madrid, et al., J. Exp. Med. 154:1517 (1981)], and dogs [Moore, et al., Tissue Antigens 36:211-220 (1990)].
The absolute molecular weights of presumed homologs from other species have been shown to vary significantly [see, e.g., Danilenko et al., Tissue Antigens 40:13-21 (1992)], and in the absence of sequence information, a definitive correlation between human integrin subunits and those identified in other species has not been possible. Moreover, variation in the number of members in a protein family has been observed between different species. Consider, for example, that more IgA isotypes have been isolated in rabbits than in humans [Burnett, et al., EMBO J. 8:4041-4047 (1989) and Schneiderman, et al., Proc. Natl. Acad. Sci.(USA) 86:7561-7565 (1989)]. Similarly, in humans, at least six variants of the metallothionine protein have been previously identified [Karin and Richards, Nature 299:797-802 (1982) and Varshney, et al., Mol. Cell. Biol. 6:26-37, (1986)], whereas in the mouse, only two such variants are in evidence [Searle, et al., Mol. Cell. Biol. 4:1221-1230 (1984)]. Therefore, existence of multiple members of a protein family in one species does not necessarily imply that corresponding family members exist in another species.
In the specific context of .beta..sub.2 integrins, in dogs it has been observed that the presumed canine .beta..sub.2 counterpart to the human CD18 is capable of dimer formation with as many as four potentially distinct .alpha. subunits [Danilenko, et al., supra]. Antibodies generated by immunizing mice with canine splenocytes resulted in monoclonal antibodies which immunoprecipitated proteins tentatively designated as canine homologs to human CD18, CD11a, CD11b and CD11c based mainly on similar, but not identical, molecular weights. Another anti-canine splenocyte antibody, Ca11.8H2, recognized and immunoprecipitated a fourth .alpha.-like canine subunit also capable of association with the .beta..sub.2 subunit, but having a unique molecular weight and restricted in expression to a subset of differentiated tissue macrophages. Antibodies generated by immunization of hamsters with murine dendritic cells resulted in two anti-integrin antibodies [Metlay, et al., J. Exp. Med. 171:1753-1771 (1990)]. One antibody, 2E6, immunoprecipitated a predominant heterodimer with subunits having approximate molecular weights of 180 kD and 90 kD in addition to minor bands in the molecular weight range of 150-160 kD. The second antibody, N418, precipitated another apparent heterodimer with subunits having approximate molecular weights of 150 kD and 90 Kd. Based on cellular adhesion blocking studies, it was hypothesized that antibody 2E6 recognized a murine counterpart to human CD18. While the molecular weight of the N418 antigen suggested recognition of a murine homolog to human CD11c/CD18, further analysis indicated that the murine antigen exhibited a tissue distribution pattern which was inconsistent with that observed for human CD11c/CD18.
The antigens recognized by the canine Ca11.8H2 antibody and the murine N418 antibody could represent a variant species (e.g., a glycosylation or splice variant) of a previously identified canine or murine .alpha. subunit. Alternatively, these antigens may represent unique canine and murine integrin .alpha. subunits. In the absence of specific information regarding primary structure, these alternatives cannot be distinguished.
In humans, CD11a/CD18 is expressed on all leukocytes. CD11b/CD18 and CD11c/CD18 are essentially restricted to expression on monocytes, granulocytes, macrophages and natural killer (NK) cells, but CD11c/CD18 is also detected on some B-cell types. In general, CD11a/CD18 predominates on lymphocytes, CD11b/CD18 on granulocytes and CD11c/CD18 on macrophages [see review, Arnaout, Blood 75:1037-1050 (1990)]. Expression of the .alpha. chains, however, is variable with regard to the state of activation and differentiation of the individual cell types [See review, Larson and Springer, Immunol. Rev. 114:181-217 (1990).]
The involvement of the .beta..sub.2 integrins in human immune and inflammatory responses has been demonstrated using monoclonal antibodies which are capable of blocking .beta..sub.2 integrin-associated cell adhesion. For example, Cd11a/CD18, CD11b/CD18 and CD11c/CD18 actively participate in natural killer (NK) cell binding to lymphoma and adenocarcinoma cells [Patarroyo, et al., J. Immunol. Rev. 114:67-108 (1990)], granulocyte accumulation [Nourshargh, et al., J. Immunol. 142:3193-3198 (1989)], granulocyte-independent plasma leakage [Arfors, et al., Blood 69:338-340 (1987)], chemotactic response of stimulated leukocytes [Arfors, et al., supra] and leukocyte adhesion to vascular endothelium [Price, et al., J. Immunol. 139:4174-4177 (1987) and Smith, et al., J. Clin. Invest. 83:2008-2017 (1989)]. The fundamental role of .beta..sub.2 integrins in immune and inflammatory responses is made apparent in the clinical syndrome referred to as leukocyte adhesion deficiency (LAD), wherein clinical manifestations include recurrent and often life threatening bacterial infections. LAD results from heterogeneous mutations in the .beta..sub.2 subunit [Kishimoto, et al., Cell 50:193-202 (1987)] and the severity of the disease state is proportional to the degree of the deficiency in .beta..sub.2 subunit expression. Formation of the complete integrin heterodimer is impaired by the .beta..sub.2 mutation [Kishimoto, et al., supra].
Interestingly, at least one antibody specific for CD18 has been shown to inhibit human immunodeficiency virus type-1 (HIV-1) syncytia formation in vitro, albeit the exact mechanism of this inhibition is unclear [Hildreth and Orentas, Science 244:1075-1078 (1989)]. This observation is consistent with the discovery that a principal counterreceptor of CD11a/CD18, ICAM-1, is also a surface receptor for the major group of rhinovirus serotypes [Greve, et al., Cell 56:839 (1989)].
The significance of .beta..sub.2 integrin binding activity in human immune and inflammatory responses underscores the necessity to develop a more complete understanding of this class of surface proteins. Identification of yet unknown members of this subfamily, as well as their counterreceptors, and the generation of monoclonal antibodies or other soluble factors which can alter biological activity of the .beta..sub.2 integrins will provide practical means for therapeutic intervention in .beta..sub.2 integrin-related immune and inflammatory responses.